1. Field of the Invention
The present invention relates to an optical disk apparatus and a tilt detection method. More specifically, the present invention relates to an optical disk apparatus and a tilt detection method which detects a tilt of an optical disk such as a DVD, a CD or the like so as to mitigate an influence of the disk tilt.
2. Description of the Prior Art
With an increased recording capacity of an optical disk and its enhanced recording density, a beam spot which irradiates to the optical disk to reproduce or record a signal is becoming minute. More specifically, in an optical disk apparatus which performs a recording, a minute beam spot is required than a reproduction-use optical disk apparatus in order to record a signal in a good condition. To obtain the minute spot, an object lens with large numerical aperture is adopted, and consequently, a side effect occurs, in which a deteriorated level of a spot quality by the disk tilt becomes evident.
The deteriorated level of a spot quality by the disk tilt mainly means a generation of a comatic aberration, that is, as a result of a blurred imagery, a spot size becomes large, and its central optical intensity deteriorates in addition thereto. If the spot size becomes large, a minute signal is not read out appropriately. In a case of the optical disk on a principal that a recording is performed by an optical heat, a decrease in the central optical intensity results in its temperature not reaching a predetermined value required for the recording, thus not possible to record. On the other hand, if an entire amount of light is increased in order to obtain the predetermined temperature, an area above the predetermined temperature expands, thus not possible to record minutely.
The disk tilt is a state produced in a case of using a disk with a large curvature. A state that a portion to which a beam irradiates is radially tilted toward the disk is referred to as a radial tilt, and a state tangentially tilted is referred to as a tangential tilt.
Referring to FIG. 1 and FIG. 2, a method of a prior art 1 in which such the disk tilt is detected and corrected is described. In FIG. 1, a disk 1, that is, a recording and reproducing body is held by a holding portion 2, rotated by a spindle motor 3a, and receives a light irradiation from an optical pick-up 4, thereby recording a signal on the disk 1 or reproducing a signal from the disk 1. The optical pick-up 4 is held by a shaft 5a, and the shaft 5a is held by a shaft holder 5b. The shaft holder 5b is fixed on a shaft holder chassis 5c. It is noted that the above-described spindle motor 3a is fixed on a spindle motor chassis 3b, and the spindle motor chassis 3b and the shaft holder chassis 5c are joined by a holding shaft 6. In addition, a cam 7 which oscillates an edge of the shaft chassis 5c up and down is provided on the spindle motor chassis 3b. 
It is noted that as shown in FIG. 2, a tilt sensor 8 which detects the tilt of the disk 1 is provided inside the optical pick-up 4. The tilt sensor 8 is an electronic part in which a light emitted from an internal LED is reflected on a reflection surface horizontal to a sensor-providing surface, and taking an electronic signal output in accordance with a position at which the reflected light falls on an internal light-receiving sensor as a reference, it is detected that the position at which the reflected light falls on the internal light-receiving sensor is deviated by a change of the output signal when the reflection surface is tilted, and consequently, the tilt on the reflection surface is detected.
The light emitted from the optical pick-up 4 is focused on the disk 1 rotated by the spindle motor 3a so as to form a minute spot. The optical pick-up 4 moves along the shaft 5a by a driving portion (not shown). Therefore, the spot is capable of scanning in a two-dimensional manner on the disk 1. This allows the optical pick-up 4 to record a signal on a signal surface provided at a depth of an inner side via a transparent cover glass layer from a surface of the disk 1, and reproduce the signal from a signal surface.
Next, descriptions are made with respect to an operation in a state that a shape of the disk 1 has a predetermined gradient toward a radial direction or in a state that the gradient is gradually changing along with a radius, and in a case that such the disk 1 is attached to an apparatus.
The tilt sensor 8 detects a radial tilt amount. The cam 7 is rotated by a driving source not shown, and oscillates the edge of the shaft holder chassis 5c up and down. As the result, the optical pick-up 4 attached on the chassis 5c, using the shaft 6 as its center, changes the gradient. It is possible to stop the cam7 by detecting a relative angle with the disk 1 by the tilt sensor 8 while changing the gradient of the optical pick-up 4 in such a state that the optical pick-up 4 and the disk 1 maintain a parallel relationship with each other. This solves the comatic aberration from the spot on the disk 1. That is, in the prior art 1, the gradient of the optical disk 4 is changed in accordance with the disk tilt amount detected by the tilt sensor 8 so as to compensate or cancel the disk tilt.
Unlike the prior art 1, there is a method that compensates or cancels the comatic aberration produced by the disk tilt by means of the comatic aberration produced by slanting the object lens. In the prior art 2, a tilt amount of the object lens capable of compensating the comatic aberration produced by the disk tilt is examined in advance, and the tilt amount of the object lens is controlled in accordance with the detected disk tilt amount.
There is a method that does not detect the tilt amount. In a prior art 3, provided is a means in which the entire optical pick-up is slanted or the object lens is slanted so that an amplitude of a waveform that the signal is reproduced, that is, a so-called RF amplitude is rendered maximum or a “jitter” showing a fluctuation of the signal with respect to time is rendered minimum.
Also is there a method which uses a signal correlated with the disk tilt out of detected signals of a reflected light in pits or emboss structure formed in advance on the disk using not only a main beam which reads the signal but also sub-beams used for a tracking. In a prior art 4, a tilt servo is performed by, while such the signal is detected, adjusting a gradient of the entire optical disk and the gradient of the object lens.
In the prior art 1, its structure becomes too complicated. In addition, it is difficult to respond to a radial tilt which changes in a time that the disk makes one rotation. In a case that a curvature of the disk varies in a period of one rotation of the disk 1, a 1-time speed DVD requires a dynamic response in a period of 45 milliseconds, and a response in a cycle of 6 milliseconds is necessary in order to respond to an 8-time speed DVD, for example. A high speed responsiveness is thus necessary for a movement in slanting the optical pick-up, and such the high speed response is difficult in the structure of the prior art 1.
The prior art 2 method is suitable for a miniaturization compared to a method which slants the entire pick-up or the disk, and in addition, its responsiveness is high. However, in realizing it, since both the disk tilt and the object lens tilt are to be detected from a common independent reference surface, thus requiring two tilt sensors. Therefore, it is difficult to miniaturize using the tilt sensor as in the prior art 1.
Furthermore, in the prior art 2, it is appropriate to detect the disk tilt only, and tilt the object lens in accordance with the detected tilt amount. However, in this case, a physical amount correlated with the tilt amount of the object lens is operated. In a case of a structure that an actuator which holds the object lens is operated by an electromagnetic force, a disequilibrium of a magnetic field is generated by manipulating a current flowing through a coil so as to slant the actuator, for example. A value of the electric current in this case is the “physical amount”. However, since this method uses an open control not a feed back control, whether or not the actuator is actually slanted by a desired degree is not clear, and in addition, a correlation relationship between the manipulated physical amount and the gradient varies on each pick-up, thus not guaranteed to be controlled in a most appropriate state.
Application of the prior art 3 is difficult to adapt in recording.
Moreover, since the method of the prior art 4 depends upon a physical format, that is, a shape of the disk, it is not possible to apply the tilt servo to all disks subject to the drive device in a compound disk drive device which performs a recording in a disk of various physical formats by a single device.